(1) Field of the Invention
The present invention relates to an impedance matching device for a silicon dioxide (SiO.sub.2) coating device and a method of impedance matching using the same, and more particularly, to an impedance matching device having a phase detecting terminal for detecting the phase of a radio frequency (RF) voltage applied to a cathode of the SiO.sub.2 coating device.
(2) Description of the Prior Art
Generally, a process of coating SiO.sub.2 compound on a bare glass is required for manufacturing a thin film transistor liquid crystal display (TFT-LCD). The coating process is conventionally carried out with a vacuum plasma coating device. As shown in FIG. 1, the vacuum plasma coating device includes an oscillator 100 which produces an RF voltage having a high frequency and amplitude, and a coating unit 300 which coats the bare glass with silicon dioxide by using the RF voltage as a power source. The voltage produced by the oscillator 100 conventionally has a frequency of 13.56 MHz and a 6 KV amplitude.
In addition, an impedance matching device 200 is provided between the oscillator 100 and the coating unit 300 to match the impedance of the oscillator 100 with the impedance of the coating unit 300, thereby increasing the efficiency of the power transmission.
As shown in FIG. 2, the conventional impedance matching device 200 (MB1) includes a coil L1 in which one terminal is connected to one of the input terminals RFIN, a first variable resistor CV1 which is connected to the input terminals RFIN in parallel, and a second variable capacitor CV2 which is provided between a coil L1 and one of the output terminals RFOUT. In operation, when the RF voltage is produced by the oscillator 100, the capacitances of the variable capacitors CV1, CV2 are varied to match the impedance of the oscillator 100 with the impedance of the coating unit 300.
In the impedance matching device 200, the first variable capacitor CV1 is provided for varying the impedance of the load. That is, the load of the coating 15 unit 300 and the impedance matching device 200. The second variable capacitor CV2, which is generally called "a tunning capacitor", is provided for varying the phase of the voltage supplied to the coating unit 300. By using the impedance matching device 200, the voltage produced by the oscillator 100 is effectively transferred to the coating unit 300.
The coating unit 300 includes four cathodes CT1-CT4 which are provided along the direction of movement of the bare glass GLS, as shown in FIG. 3. The silicone dioxide compound is initially adhered on the cathodes CT1-CT4. The case of the coating unit 300 acts as an anode. Ar gas is provided in the case for inducing the discharge between the anode and the cathodes CT1-CT4. In operation, when the bare glass GLS is moved through the coating unit 300, the cathodes are sequentially discharged to coat the bare glass GLS with the silicon dioxide.
When coating the bare glass GLS, the timing of the discharging operations of the cathodes should be accurately controlled. As shown in FIG. 4, the discharging operation timing of the first cathode CT1 and the discharging operation timing of the second cathode CT2 should be controlled so that the phase difference of the voltages on the first and the second cathodes CT1, CT2 is 180.degree.. In addition, the phase difference of the voltages on the second and the third cathodes CT2, CT3 should be maintained to 180.degree., and the phase difference of the voltages on the third and the fourth cathodes CT3, CT4 should also be maintained to 180.degree..
If the discharging operation timing of the cathodes CT1-CT4 are not controlled as described above, the silicon dioxide can not be uniformly coated on the bare glass GLS, and the quality of the TFT-LCD is deteriorated. For example, if the second cathode CT2 and the third cathode CT3 are operated concurrently, an arc discharge phenomenon occurs and many pinholes are formed on the coating layer of the glass GLS. Thus, to prevent pinholes from forming on the coating layer, the phase of the voltages on the cathodes CT1-CT4 should be monitored, and the phase difference of voltages on the cathodes CT1-CT4 should be controlled precisely to 180.degree.. For that purpose, a separate probe for detecting the phase is conventionally used. That is, the separate probe is inserted into the coating unit 300 to detect the phase of the voltage, and an oscilloscope is used to analyze the phase of the detected voltage. According to the detected phase, the second variable capacitor CV2 is adjusted to vary the phase of the voltage supplied to the coating unit 300.
However, in order to use the probe, the probe must be inserted into the coating unit 300 manually. That is, an operator must open the cap of the coating unit 300 and insert the probe into the coating unit 300. Thus, it is troublesome for the operator, and furthermore, the operator is liable to receive an electrical shock due to the high voltage supplied to the coating unit 300.